Vehicles traveling above the atmosphere are frequently impacted by micrometeorites, which often collide at relative velocities as great as 30 kilometers per second. Fortunately, most micrometeorites have masses considerably less than 1 gram and comprise minerals such as silicates that are characteristically low in density. The combination of low density and hypersonic velocities render them ideal for destruction by structures generally termed Whipple shields.
Frederick Whipple, an astronomer, proposed a concept in the 1940s comprising a thin metal sheet supported in a manner such that a gap would be maintained between the thin shield and structure requiring protection. Micrometeorites impacting the thin metal shield would melt or vaporize, as would the part of the shield in contact with the projectile. Molten material and vaporized debris would then be unable to puncture the structure beyond the gap. Numerous tests in the 1960s proved the efficacy of the concept. As a result, Whipple shields were used to protect Apollo spacecraft and the International Space Station.
Conventional Whipple shields require that the mass of the thin shield be small so that the entire portion interacting with the projectile will melt or vaporize, and thereby be incapable of piercing the structure requiring protection once it is dislodged from the rest of the shield. Such a thin metal layer will be penetrated easily by larger and denser objects. This unfortunate situation results from the hydrodynamic nature of projectile impacts at velocities much greater than 2 kilometers per second. Shear stresses generated in shields by impact at hypersonic velocities greatly exceed the mechanical strength of any material.
Spacecraft are not only at risk from meteorite impacts. Vehicles in orbit around the Earth may collide with one of many thousands of man-made objects. Such objects may be small items such as tools, gloves, or bolts. Entire assemblies also pose a hazard, such as shrouds, rocket motor casings and empty metal tanks. Relative velocities between orbiting debris and spacecraft may be considerably lower than 30 kilometers per second, but the greater density and larger mass of such objects would readily penetrate any Whipple shield made with the present art.
Terrestrial vehicles and structures typically employ heavy armor assemblies to resist penetration by dense, supersonic projectiles. Armors made with the present art generally use thick, dense metal plates and strong ceramic facings to erode and break up dense projectiles. Although bulky and heavy, this kind of armor assembly is generally effective against dense projectiles impacting at velocities less than 10 kilometers per second.
Against superplastic projectiles formed by explosive devices, a different kind of assembly generally termed “reactive armor” is often employed. Reactive armor assemblies comprise two thick, metal plates sandwiching an explosive. Reactive armor assemblies are placed at an angle to the anticipated direction of projectile approach. Penetration by a superplastic metal penetrator, typically called a shaped charge jet, detonates the explosive. Detonation causes the two metal plates to move in opposite directions, thereby disrupting the shaped charge jet and rendering it incapable of piercing armor behind the assembly.
Thick armor and reactive armor assemblies are far too heavy for use aboard spacecraft. Heavy armor serves no other useful purpose, so the rocket size required to launch the extra mass of terrestrial vehicle armor would impose an expensive burden. Such a burden would displace weight and space that would otherwise be available for fuel, provisions and scientific equipment.
Improved means of protecting spacecraft against impact by dense objects moving at high velocity are highly desirable. Many advantages would accrue if such protection means can be provided with much less mass and bulk compared with armor made using the present art.